Polyvinyl alcohol polymers are not thermoplastic resins processable by conventional melt spinning, injection molding or melt extrusion. When subjected to heat, polyvinyl alcohol decomposes readily before reaching its melt temperature. Existing art shows that continuous multifilament yarns have been spun by solution spinning or wet spinning processes wherein polyvinyl alcohol is dissolved in a suitable solvent, usually water, forced through a spinnerette and the filaments coagulated in a precipitating bath.
One U.S. patent directed to this approach is U.S. Pat. No. 3,365,527 which describes a method for spinning polyvinyl alcohol fibers into a caustic coagulating bath. Polyvinyl alcohols that are low water swelling are preferred.
U.S. Pat. No. 3,660,556 provides a similar process wherein an aqueous polyvinyl alcohol solution, containing a water soluble boric acid compound adjusting the pH to 3 to 5, is coagulated in a caustic bath having a pH of 13.5 to 14.
Solution processing suffers several limitations. First, the filament formation by precipitation out of an aqueous solution requires a polymer with a limited water solubility. It has not been possible heretofore to produce large diameter monofilament without substantial failure. In addition, such a process requires very costly equipment which can only be profitable if operated on a very large scale.
High temperature extrusion of polyvinyl alcohol films is known, however. One U.S. patent directed to this approach is U.S. Pat. No. 3,607,812 which describes a method for extruding a mixture of polyvinyl alcohol resin having a degree of hydrolysis of at least 97 percent and containing sodium acetate, and a polyhydric alcohol plasticizer compatible with the resin. The resulting films are insoluble in water at temperatures below 40.degree. C., but readily soluble in warmer water.
U.S. Pat. No. 3,985,849 is directed toward a method for preparing a biaxially oriented polyvinyl alcohol film by extruding a mixture of polyvinyl alcohol resin, water and optionally a polyhydric alcohol plasticizer. The polyvinyl alcohol resin has a degree of polymerization of between 1100 and 2000 and a degree of hydrolysis of not less than 98 percent.
U.S. Pat. No. 4,206,101 discloses a composition suitable for use in the preparation of extrudable cold water soluble films comprising a partially hydrolyzed low molecular weight polyvinyl alcohol and a polyethylene glycol. The polyethylene glycols must have an average molecular weight in the range of about 325 to 550, those with higher or lower molecular weights were reported to be unsatisfactory. The patent also states that the use of glycerin as the plasticizer will yield an unsatisfactory composition.
Thus, while certain techniques have been developed to produce polyvinyl alcohol films, the art of which I am aware has not provided a process for producing the desired water soluble, oriented, continuous polyvinyl alcohol monofilament.